Auto-zero offset operational (buffer) amplifiers are a class of analog circuits that combine analog and digital (switching) circuitry resulting in very low input-referred DC offset and noise. These operational amplifiers are often used in precision applications where high gain is necessary to resolve very small voltages. Examples include RTD, thermocouple, resistive current measurement and other sensing applications. The use of an auto-zero offset operational amplifier may relax the accuracy requirements of the A/D converter, saving cost. Today's auto-zero offset operational amplifiers bear little resemblance to the early chopping schemes that were used to reduce the average offsets. Those circuits were very simple using discrete amplifiers and switches to chop the amplifier's inputs and outputs using a clock. Heavy filtering was required to achieve low offset, and filter out the switching noise. Chopper amplifiers had a low frequency bandwidth, usually a few Hertz, limited by the large settling time constants.
Referring to FIG. 1, depicted is a schematic block diagram of a prior art auto-zero offset operational amplifier. The prior art auto-zero offset operational amplifier, generally represented by the numeral 100, may comprise at least two amplifiers 102 and 104 that are combined to generate the final signal output, Vout. A conventional wide bandwidth “main” amplifier 102 is connected directly from input to output and continuously processes the incoming signal. A second, very high gain “nulling” amplifier 104 is connected in parallel for offset correction. The nulling amplifier 104 is zeroed to null its own offset and to eliminate low frequency 1/f noise below the clock frequency of the clock oscillator 110. Zeroing involves shorting together the two inputs of the amplifier 104 with switch 108 and storing the resulting offset onto a capacitor 112. During a compensation cycle, this correction voltage is applied to the nulling amplifier 104 through an auxiliary port 116. The correction voltage is held on a main amplifier auxiliary port 118 by a storage capacitor 114 when the nulling amplifier 104 is disconnected by switch 106 from the main amplifier 102 during its zeroing cycle. This correction voltage is then used to null the offset of the main amplifier 102 through its auxiliary port 118.
Early auto-zero offset amplifiers combined a wideband “main” amplifier and a single “nulling” amplifier. The single nulling amplifier has a sample and hold to correct its own offset, and to reduce the offset of the main amplifier. Early implementations required external capacitors and had sampling frequencies of a few hundred Hertz. Great improvements have been made in the state of the art over the years. Modern auto-zero offset operational amplifiers are now able to achieve DC offsets of a few microvolts with very low temperature drift.
However, due to the internal clock switching in the nulling amplifier, some switching noise will appear at the output. This will be most predominant around the sampling clock frequency. If this noise is not symmetric, i.e., generating substantially equal amounts of positive and negative glitches, an average DC offset may result in the system. Therefore, reduction of these glitches is essential for good DC performance.
An auto-zero offset buffer amplifier having too small auto-zero capacitance develops high switching errors. In order to reduce the switching errors to a smaller value, higher value capacitance (larger area capacitors) are required in the auto-zero circuit. However, increasing the capacitance requires more integrated circuit silicon area for the capacitors. For example, with a specific requirement of +/−5 mV maximum switching errors, the capacitors would be ten (10) times larger and the overall module on the integrated circuit die would be at least two times bigger.